The invention described herein relates generally to optically pulsed electron accelerators and more particularly to optically pulsed electron accelerators for use as improved injectors for free electron lasers. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
The free electron laser was first described by John Madey of Stanford in 1971. It is widely acknowledged that the free electron laser is of great potential value in the fields of medicine, spectroscopy, ir imaging, chemical processing, welding, laser fusion, communications and directed energy weapons. J. T. Riordan, "The Free-Electron Laser: Medicine's Rising Star," Photonics Spectra 40 (July 1983). R. B. Hall, "Lasers In Industrial Chemical Synthesis," Laser Focus 57 (September 1982). Compared to conventional lasers, free electron lasers provide broad tunability, excellent spot size control, excellent pulse width control and high power density and control.
A free electron laser includes an injector for providing a source of high energy electrons to be accelerated to higher energy levels to serve as the source of energy transferred to an optical output beam in the free electron laser. To operate efficiently, a free electron laser requires an injector that can simultaneously provide high peak current, short electron burst duration and high beam quality. A high quality beam is a low emittance beam, i.e., a beam with little motion in directions other than the desired direction of beam travel. Conventional injectors can not simultaneously achieve these three characteristics. Consequently, it is highly desirable to develop an injector that can simultaneously achieve these three characteristics.
A conventional electron accelerator (injector) for use with a free electron laser is powered by a radio frequency (rf) energy source. It comprises an electron gun, a buncher section whose purpose is to increase the system efficiency, and the accelerator proper. The electron gun usually consists of a thermionic electron emitter which may deliver a continuous current or it may be pulsed to deliver short bursts of electrons at widely varying repetition rates. The buncher section usually consists of one or more accelerating cavities whose function is to modulate the velocity of the electron stream in a sinusoidal manner with the result that at a position downstream from the buncher cavities, the electron current is periodically bunched or peaked. The periodicity of the bunches is harmonically related to the period of the rf energy source for the linear accelerator. An rf powered linear accelerator consists of a series of resonant cavities in which large sinusoidal electric and magnetic fields are established by the flow of rf energy into the cavity. The electron bunches are injected with the proper phase to be accelerated periodically by the rf electric field. In a traveling wave accelerator the electrons ride the crest of the electric field through successive cavities. In a standing wave accelerator, the electron bunches are shielded from the periodic decelerating fields by metallic drift tubes. Prior experience with conventional electron accelerators has demonstrated that a degradation of beam quality occurs in the bunching system. The optically pulsed electron accelerator of the present invention avoids this degradation by eliminating the bunching system.
U.S. Pat. No. 4,313,072 to Wilson et al. discloses a light modulated electron beam driven radio-frequency emitter for power generation. Pulses of light impinge on a photoemissive device which generates ane electron beam having the characteristics of the light pulses. However, the device described in Wilson et al. is designed to extract the energy from the accelerated electron beam so generated as radio frequency emission. Moreover, the electron beam is generated outside of the cavity and accelerated using dc electric fields applied between the photoemitter and the energy extraction cavity.